SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING LESSON TWO MACHINING ACCURACY Product quality is always a matter of prime importance in manufacturing. It should be given parameter the highest priority in process planning. The 参数,参量 quality of a machined part can be expressed in coordination geometrical parameters( dimension, shape,相互位置 coordination and surface finish, etc. ), physical surface finish parameters (electrical conductivity, thermal 表面光洁度 conductivity, magnetic conductivity, etc.) chemical parameters(corrosion resistance. etc. and mechanical parameters(strength, hardness, etc.). The requirements of these parameters of a part are determined by the designer, according to the condition under which the product is to function.(c)
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING LESSON TWO MACHINING ACCURACY Product quality is always a matter of prime importance in manufacturing. It should be given the highest priority in process planning. The quality of a machined part can be expressed in geometrical parameters (dimension, shape, coordination and surface finish, etc.), physical parameters (electrical conductivity, thermal conductivity, magnetic conductivity, etc.), chemical parameters (corrosion resistance. etc.) and mechanical parameters (strength, hardness, etc.). The requirements of these parameters of a part are determined by the designer, according to the condition under which the product is to function.(c) coordination 相互位置 surface finish 表面光洁度 parameter 参数，参量
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING (cThe degree of coincidence of the real parameters obtained after a part is manufactured, with the parameters defined in part design, represents the manufacturing quality of the part. For the convenience of anal ysis, the degree of coincidence between the real macro-geometrical parameters(dimension, shape and coordination) of a part obtained after machining, and those specified in part design is defined as machining accuracy. The degree of coincidence between the real micro-geometrical parameters(surface finish)and the parameters of the physic-mechanical properties of part surfaces, obtained after machining a part, with those specified in part design is defined as sur face quality Other physical and chemical parameters are involved only in some special cases
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING (c)The degree of coincidence of the real parameters obtained after a part is manufactured, with the parameters defined in part design, represents the manufacturing quality of the part. For the convenience of analysis, the degree of coincidence between the real macro-geometrical parameters (dimension, shape and coordination) of a part obtained after machining, and those specified in part design is defined as machining accuracy. The degree of coincidence between the real micro-geometrical parameters (surface finish) and the parameters of the physic-mechanical properties of part surfaces, obtained after machining a part, with those specified in part design is defined as surface quality. Other physical and chemical parameters are involved only in some special cases
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING Machining accuracy is expressed quantitatively by the magnitude of machining error. Machining error is defined as the difference between the parameters of a machined part and that of a perfect (absolutely accurate) part specified in the part drawing. The machining error varies from part to part even in the same batch of production. Although machining errors al ways exist in machining processes, a part can be considered acceptable, provided the magnitude of machining errors do not 公差 exceed the tolerance limits. The tolerance represents the maximum allowable machining error. The required machining accuracy of a part is specified by the designer, while the machining accuracy requirements of the part in each machining stage are determined in accordance with the needs and accuracy of the process by the process planner
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING Machining accuracy is expressed quantitatively by the magnitude of machining error. Machining error is defined as the difference between the parameters of a machined part and that of a perfect (absolutely accurate) part specified in the part drawing. The machining error varies from part to part even in the same batch of production. Although machining errors always exist in machining processes, a part can be considered acceptable, provided the magnitude of machining errors do not exceed the tolerance limits. The tolerance represents the maximum allowable machining error. The required machining accuracy of a part is specified by the designer, while the machining accuracy requirements of the part in each machining stage are determined in accordance with the needs and accuracy of the process by the process planner. 公 差
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING Parameters of Machining Accuracy and Surface Quality Machining accuracy is associated with a number of macro-geometrical parameters andflatness can be classified as follows 平直度,平面度 1. Machining accuracy of part sur faces roundness Accuracy of dimensions of part sur faces(for‖圆度,球度 example. accuracy of the diameters of cylindrical surfaces, accuracy of the diameters cylindricity of spherical surfaces, and accuracy of the圆柱度 angles at the top of the cones, etc.) Accuracy of shapes of part surfaces(for example, flatness of a plane, roundness, and cylindricity of a cylindrical sur face, etc
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING Parameters of Machining Accuracy and Surface Quality Machining accuracy is associated with a number of macro-geometrical parameters and can be classified as follows: l. Machining accuracy of part surfaces. - Accuracy of dimensions of part surfaces (for example, accuracy of the diameters of cylindrical surfaces, accuracy of the diameters of spherical surfaces, and accuracy of the angles at the top of the cones, etc.). - Accuracy of shapes of part surfaces (for example, flatness of a plane, roundness, and cylindricity of a cylindrical surface, etc.). flatness 平直度,平面度 roundness 圆度，球度 cylindricity 圆柱度
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING 2. Machining accuracy of relative positions between part surfaces Accuracy of dimensions between part parallelism 平行度 surfaces(for example, the distance between two parallel planes or the centers of two holes, etc.) perpendicularity 垂直度 Accuracy of positional relations between part surfaces(for example, the parallelism and surface roughness perpendicularity of two planes or two axes, etc.) 表面粗糙度 The surface quality of a part indicates the surface characteristics of the part. It includes either geometrical or physic-mechanical parameters, which are listed below D Surface roughness 2) Physic-mechanical state of surface layer
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING 2. Machining accuracy of relative positions between part surfaces. - Accuracy of dimensions between part surfaces (for example, the distance between two parallel planes or the centers of two holes, etc.). - Accuracy of positional relations between part surfaces (for example, the parallelism and perpendicularity of two planes or two axes, etc.). The surface quality of a part indicates the surface characteristics of the part. It includes either geometrical or physic-mechanical parameters, which are listed below: l) Surface roughness. 2) Physic-mechanical state of surface layer. parallelism 平行度 perpendicularity 垂直度 surface roughness 表面粗糙度
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING Among the various parameters of machining accuracy, the shape accuracy of part surfaces relies on the cutting tools, and the relative motions between cutting tools and workpieces The dimensional accuracy, including accuracy of dimensions of part surfaces themselves and dimensions between part surfaces, can be achieved through different method Methods for Obtaining required Dimensional Accuracy In machining, different measures can be adopted to attain the required dimensional accuracy. There are basically two different methods: trial cutting (or grinding) and automatic dimension maintenance Trial Cutting 试切法 Trial cutting achieves the required dimensional accuracy in several trial cutting(or grinding) passes. A certain length of the surface to be machined is first cut and measured. (c)
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING Among the various parameters of machining accuracy, the shape accuracy of part surfaces relies on the cutting tools, and the relative motions between cutting tools and workpieces. The dimensional accuracy, including accuracy of dimensions of part surfaces themselves and dimensions between part surfaces, can be achieved through different method. Methods for Obtaining Required Dimensional Accuracy In machining, different measures can be adopted to attain the required dimensional accuracy. There are basically two different methods: trial cutting (or grinding) and automatic dimension maintenance. Trial Cutting Trial cutting achieves the required dimensional accuracy in several trial cutting (or grinding) passes. A certain length of the surface to be machined is first cut and measured.(c) 试切法
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING (c) The cutting tool position with respect to the workpiece is then adjusted in the next cutting pass. The process repeats until the machined dimension falls into the specified tolerance limits After that, the entire length of the surface is machined in a single pass and the dimensional accuracy can be ensured. For example, when turning a shaft on a lathe, in order to obtain the diameter d and length I with their required accuracy, several trial cutting passes are first performed on a certain length of the cylindrical surface. After each cut, the diameter is measured and the turning tool is then adjusted. When the measured diameter enters its tolerance limits. the entire length of the cylindrical surface is turned with the automatic feed. Before the tool reaches the stepped face, the continuous feed should be stopped and the trial cutting and measuring are performed until the dimension of length l enters its tolerance limits
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING (c) The cutting tool position with respect to the workpiece is then adjusted in the next cutting pass. The process repeats until the machined dimension falls into the specified tolerance limits. After that, the entire length of the surface is machined in a single pass and the dimensional accuracy can be ensured. For example, when turning a shaft on a lathe, in order to obtain the diameter d and length l with their required accuracy, several trial cutting passes are first performed on a certain length of the cylindrical surface. After each cut, the diameter is measured and the turning tool is then adjusted. When the measured diameter enters its tolerance limits, the entire length of the cylindrical surface is turned with the automatic feed. Before the tool reaches the stepped face, the continuous feed should be stopped, and the trial cutting and measuring are performed until the dimension of length l enters its tolerance limits
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING Trial cutting is characterized by its low production efficiency and is not suitable for large volume production However. using this method. the machine tool does not need to be preset in advance. Moreover, the effect of dimensional variation of the blanks(shape, sizes and hardness of the material, etc. can be reduced in machining operations. For these reasons, it has been widely used in traditional small batch production The machining accuracy of the trial cutting method depends mainly upon the minimum possible thickness of the chip removal in the last trial cutting pass and the skill level of the operator. That is the reason why it is easier to attain higher machining accuracy by using a grinding operation than a turning operation
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING Trial cutting is characterized by its low production efficiency and is not suitable for large volume production. However, using this method, the machine tool does not need to be preset in advance. Moreover, the effect of dimensional variation of the blanks (shape, sizes and hardness of the material, etc.) can be reduced in machining operations. For these reasons, it has been widely used in traditional smallbatch production. The machining accuracy of the trial cutting method depends mainly upon the minimum possible thickness of the chip removal in the last trial cutting pass and the skill level of the operator. That is the reason why it is easier to attain higher machining accuracy by using a grinding operation than a turning operation
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING Automatic Dimension maintenance This method automatically ensures thereamer绞刀 attainment of the required machining accuracy without the need of trial cutting, broaches fiLTJ and measuring and correction of the cutting tool position tap丝锥,螺丝攻 There are different ways to automaticall obtain the required machining accuracy 1. Use of fixed dimension cutting tools. The required machining accuracy is obtained by using cutting tools with fixed shape and dimensions. such as reamers. broaches and taps
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING Automatic Dimension Maintenance This method automatically ensures the attainment of the required machining accuracy without the need of trial cutting, and measuring and correction of the cutting tool position. There are different ways to automatically obtain the required machining accuracy. l. Use of fixed dimension cutting tools. The required machining accuracy is obtained by using cutting tools with fixed shape and dimensions, such as reamers, broaches and taps. reamer 绞刀 tap 丝锥，螺丝攻 broaches 拉刀
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING 2. Machining on preset machine tools. The positions preset of cutting tools and/or stoppers relative to the‖预调,预置 workpiece are preset and fixed on the machine tool stopper These positions remain unchanged in machining an定程器 entire batch of workpieces. Before machining, the挡块 turning tool and the stopper are preset according to multi-cut the required diameter d and length I with their ashes tolerances. Keeping the positions of the turning tool 多刀车床 and the stopper unchanged, the machining accuracy turret lathes of the batch of workpieces can be achieved in the 转塔车床 operation Many types of machine tools have been designed planer-type for this purpose, such as multi-cut lathes, turret lathes, milling planer-type milling machines and some automatic machines machine tools 龙门铣床
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING 2. Machining on preset machine tools. The positions of cutting tools and/or stoppers relative to the workpiece are preset and fixed on the machine tool. These positions remain unchanged in machining an entire batch of workpieces. Before machining, the turning tool and the stopper are preset according to the required diameter d and length l with their tolerances. Keeping the positions of the turning tool and the stopper unchanged, the machining accuracy of the batch of workpieces can be achieved in the operation. Many types of machine tools have been designed for this purpose, such as multi-cut lathes, turret lathes, planer-type milling machines and some automatic machine tools. preset 预调,预置 stopper 定程器 挡块 multi-cut lathes 多刀车床 turret lathes 转塔车床 planer-type milling machines 龙门铣床
